1. Field of the Invention
The invention relates to a system for controlling and protecting electrically driven pumps for liquids and is particularly directed to an apparatus and method for the control and protection of battery powered marine macerator pumps.
2. Description of the Prior Art
Local, state and federal marine regulations focusing on ecological considerations have imposed restrictions on the use of recreational and commercial boats to prevent discharge of unprocessed waste into the ocean and waterways. These requirements have led to an increased use of marine macerator pumps. Macerator pumps pulverize the refuse from onboard heads or holding tanks and pump it overboard. These pumps are typically turned on manually when it is desired to empty the refuse holding tank and run until manually shut off. This manual shut off requirement introduces the possibility of damage to the pump. A common occurrence is for the operator to turn the macerator on, become involved in other on-board activities or go ashore, forgetting to turn the macerator off. This results in unnecessary battery drain, excessive pump wear, and possible motor burnout. Another potential problem occurs when the pump is jammed and/or when the motor is inadvertently left on resulting in stall current flowing through the motor windings for an excessive time and causing damage to the motor and/or the switch.
Because of the expense, labor and inconvenience involved in replacing a macerator pump, a method of automatically sensing motor operating conditions and completion of the disposal operation to control the pump is desirable. It is recognized that electro mechanical indicators such as floats are used in some arts for detecting when a tank is empty. Such an arrangement is not generally acceptable for macerator pumps since these indicators are subject to failure due to adverse environmental conditions. Failure of such devices to properly detect completion of the pumping process can allow the pump to operate at no load leading to overheating which may result in damage to the pump. It is also desirable to detect when a pump is jammed to shut off the motor to prevent operation at stall currents and the consequent damage of the motor and pump due to overheating.
Motor controls have been proposed in other arts to detect motor load. It has been proposed to monitor the work performed by a pump by sensing various electrical parameters, such as motor current. With this approach, the sensing and control is accomplished in a benign environment. Applications of this method of pump sensing and control are typically found in the oil producing industry and in water, sump and marine bilge pump control systems. Methods and apparatus of the type to control and protect oil well pumps and increase production are described in U.S. Pat. No. 4,473,338, Victor H. Garmong, Sep. 25,1984; U.S. Pat. No. 5,015,151, Dale R. Snyder et al, May 14, 1991; and U.S. Pat. No. 5,284,422, John M. Turner, Feb. 8, 1994. Methods and apparatus of the type to control and protect water, sump and marine bilge pumps are described in U. S. Pat. No.; 4,841,404, William W. Marshall et al, Jun. 20,1989, and U.S. Pat. No. 5,324,170, William N. Anastos, Jun. 28, 1994. These systems typically act continuously to pump off an essentially continuously replenished supply of liquid. Such systems are generally rather elaborate and are installed in fluid pump systems far exceeding the cost of marine pulverizing systems. Furthermore, such systems to not typically provide protection against a sensing system failure which would allow the pump to continue to run after the work was completed in a stalled or dry condition.
There exists a need for a marine pulverizer motor control which is inexpensive and convenient to install. With the enactment of marine regulations requiring pulverizers for the majority of recreational boats having heads onboard, the demand for pulverizer controls has grown significantly. Pulverizers used in recreational boats are generally inexpensive and any practical control therefor must also be relatively inexpensive. Since the holding systems for many such boats already incorporate pulverizers, it is important that any control be convenient to retrofit into the circuitry of a previously installed pump motor. It would also be desirable that any such control be contained in a compact housing having connectors readily connectable to the circuitry of the pump motor without a need for a high skilled installation technician.
The present invention provides an electrical control and protection module which can be economically and easily installed in a marine dc motor driven macerator pump power circuit. The module responds to a manual start command to automatically control the pump throughout the disposal operation and to remove pump power when disposal has been completed and the refuse holding tank is empty. It will be appreciated that, upon full evacuation of the holding tank, the unloaded pump will run free thus reducing the load on the motor causing the current drawn by such motor to assume its lowest level to, when motor current levels fall outside a pre-set range to shut the motor off. On the other hand, when the pump encounters undue resistance, as in a clogged impeller, the current to such motor will rise to a stall condition thus presenting a risk of overheating the motor or associated circuitry. We have discovered that these current conditions in the motor can be sensed to cause the module to control motor operation.
The module also includes a backup timer to monitor elapsed pump operating time and terminate pump operation when the elapsed operating time exceeds the maximum time required to empty the holding tank. This serves to protect the system against a sensing failure which otherwise might allow uninterrupted pump operation leading to overheating and damage.
Motor manufacturers typically publish specifications setting the maximum and minimum sustained current level for the particular motor. Such motors typically start at stall current levels, which exceed the maximum allowable sustained current level for a very short time, but then quickly accelerate building up back EMF which tends to oppose current flow dropping the operating current down to a nominal operating level. At start up, a macerator pump requires a short period of time to stabilize during which time the motor current may drop below minimum allowable sustained current levels until nominal flow levels are reached. As the holding tank is emptied and the pumping load is reduced due to liquid starvation, the motor current will also drop below minimum allowable current levels. Occasionally air bubbles will pass through the pump momentarily reducing the pumping load and also the operating current. To accommodate these characteristics of pumps and motors, and avoid premature shutdown of the disposal operation, several features have been provided by a preferred embodiment of the control module of this invention.
The preferred embodiment control module contains firmware programmable to set maximum and minimum sustained motor current limits, maximum motor operating time, and predetermined delays to momentarily delay unwanted motor shutdown. In this regard, it will be appreciated that motor current will surge both at start up and when the pump is momentarily unloaded during operation as by the passage of an air plug or air bubbles through the pump impeller during the macerating process. It will also be appreciated that as the holding tank approaches full evacuation and the macerating process is being completed, the load on the pump will be decreased tending to reduce motor current which might initiate shut down. To prevent premature shut down, the module incorporates time delays responsive to the momentary current drop due to the encounter of air bubbles and to current drop at the end of the cycle to thus avoid premature shut down before full evacuation and purging of the fluid lines connected with the pump.
Some embodiments of the control module include voltage regulation equipment to regulate the on board battery voltage down to 5VDC for logic circuitry, a RAM, a real time current sensor device, a comparator to determine when the sensed real-time current exceeds the time normally required to macerate a full load.
The module may be pre-programmed by the manufacturer for various ranges of pump motor power and holding tank size. The appropriate module may be selected by the boat owner for his or her particular configuration. This renders the control module design adaptable for a wide range of DC motor pump applications and enables production of low cost, easily installed macerator pump control and protection modules.